Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($)	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net income	$ 76,915,000	$ 51,463,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	63,580,000	54,082,000
Accretion of asset retirement obligation	1,068,000	919,000
Stock-based compensation	3,823,000	5,343,000
Amortization of deferred lease income	(1,342,000)	(1,343,000)
Income attributable to sale of tax benefits, net of interest expense	(8,303,000)	(6,844,000)
Equity in losses (earnings) of investees	(1,598,000)	2,027,000
Mark-to-market of derivative instruments	1,499,000	(2,462,000)
Loss on disposal of property, plant and equipment	4,942,000
Write-off of unsuccessful exploration activities	119,000	0
Gain on severance pay fund asset	721,000	(1,537,000)
Deferred income tax provision and deferred charges	(5,060,000)	34,771,000
Liability for unrecognized tax benefits	747,000	395,000
Deferred lease revenues	(205,000)	(182,000)
Proceeds from insurance recoveries	(7,150,000)
Changes in operating assets and liabilities, net of amounts acquired:
Receivables	2,977,000	(625,000)
Costs and estimated earnings in excess of billings on uncompleted contracts	(5,628,000)	(7,703,000)
Inventories	(981,000)	(103,000)
Prepaid expenses and other	433,000	1,820,000
Deposits and other	6,000	652,000
Accounts payable and accrued expenses	(54,183,000)	(4,636,000)
Billings in excess of costs and estimated earnings on uncompleted contracts	4,105,000	14,056,000
Liabilities for severance pay	(982,000)	2,425,000
Other long-term liabilities	(243,000)	(248,000)
Net cash provided by operating activities	67,050,000	114,158,000
Cash flows from investing activities:
Capital expenditures	(139,125,000)	(116,015,000)
Investment in unconsolidated companies	(3,800,000)	(27,412,000)
Cash paid for acquisition of controlling interest in a subsidiary, net of cash acquired	(95,093,000)	(35,300,000)
Proceeds from insurance recoveries related to destroyed equipment	1,488,000
Decrease (increase) in severance pay fund asset, net of payments made to retired employees	340,000	(130,000)
Net cash used in investing activities	(236,190,000)	(178,857,000)
Cash flows from financing activities:
Proceeds from long-term loans, net of transaction costs	100,000,000
Proceeds from exercise of options by employees		785,000
Proceeds from the sale of limited liability company interest in Tungsten, net of transaction costs	4,134,000	2,017,000
Proceeds from revolving credit lines with banks	1,791,400,000	437,500,000
Repayment of revolving credit lines with banks	(1,684,300,000)	(407,500,000)
Repayments of long-term debt	(28,264,000)	(33,177,000)
Cash paid to noncontrolling interest	(8,030,000)	(14,594,000)
Payments of capital leases	(972,000)	(751,000)
Deferred debt issuance costs	(1,428,000)	(3,731,000)
Cash dividends paid	(16,702,000)	(12,426,000)
Net cash provided by (used in) financing activities	188,241,000	(31,877,000)
Net change in cash and cash equivalents and restricted cash and cash equivalents	19,101,000	(96,576,000)
Restricted cash and cash equivalents acquired in a business combination	26,993,000
Cash and cash equivalents and restricted cash and cash equivalents at beginning of period	96,643,000	264,476,000
Cash and cash equivalents and restricted cash and cash equivalents at end of period	142,737,000	167,900,000
Supplemental non-cash investing and financing activities:
Increase (decrease) in accounts payable related to purchases of property, plant and equipment	(6,202,000)	2,338,000
Accrued liabilities related to financing activities	1,979,000
Tungsten [Member]
Cash flows from financing activities:
Proceeds from the sale of limited liability company interest in Tungsten, net of transaction costs	$ 32,403,000